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furnace fan limit switch control switch (C) Daniel FriedmanFAQs: Furnace Blower Fan Cycles On / Off After Call for Heat
Questions & answers help diagnose & fix furnace blower fan cycling after heat

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FAQs about why a furnace turns on after call for heat has ended:

These questions & answers help diagnose & fix a heating furnace blower fan that cycles on and off after the call for heat has stopped. For safety this furnace fan cycling problem should be diagnosed and repaired promptly. This article is part of a series of heating furnace blower fan diagnosis & repair procedures given free at InspectApedia.com. Content suggestions are always welcomed.



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Questions & Answers: Why Does a Blower Fan Keep Cycling On/Off After Call for Heat Has Stopped

furnace fan limit switch control switch (C) Daniel Friedman

Question: Long blower on time on Luxaire Furnace

(May 18, 2014) Gordon said:
My Luxaire Furnace works but when it starts cool down the blower stays on for a hour
then turns off and stays off.

Reply:

If the blower stays on for a long time but then turns off on its own AFTER the burner has shut down I suspect that the air filter is dirty, the blower is dirty, or ductwork is crimped or disconnected - something is restricting air flow so that not only does it take a long time for enough air to pass through the heating plenum to cool it down, you are also unnecessarily increasing heating costs.

Question: success at getting long-on blower time to work properly by adjusting the FAN OFF temperature on the limit switch

(Nov 7, 2014) TheFremontMan said:
My blower used to stay on for more than an hour after the burner turned off. It turned out that the lowest setting (fan-off temperature) was set around 60 F.

I adjusted it to 90 F as suggested in this article, and the fan turned off two minutes after the burner turned off.

Reply:

Wow that sounds right.

That after-blow cycle is designed to cool down the heat exchanger to reduce the risk of cracks that might occur if the blower stopped while the furnace heat exchanger were still quite hot. Surely an hour is more than needed to cool down the equipment. If you can find a shut-down temperature recommendation from your furnace manufacturer or from the limit swith manufacturer we'd end up spot-on.

Question: gas furnace turns off before coming up to full temperature

(Dec 23, 2014) brent hewitt said:
my gas fired furnace will kick on but not come up to full temp before it turns off. When I turn off thermostat and back on it kicks on and does reach temp and stay there until the next day. The temp never goes below the evening lowered temp when i get up the next morning. The thermostat is new & I replaced a relay and they are all working. This only occurs in 1 of my zones

Reply:

Brent

The limit switch turns off a furnace if the plenum temperature is too high - risking damage to the equipment.

Look for a dirty or blocked air filter.

(Dec 29, 2014) brent lewis said:
i replaced filter & appears to be working better, getting cold the next few days, so that will be a good test

combination furnace control Honeywell L4064B with cover on

Question: blower never shuts off

(Jan 4, 2015) Anonymous said:
my furnace burner goes on and reaches the heat setting then turns off but the blower never shuts off it runs continuously and i replaced the thermostat but made no difference

Reply:

Anon:

In the article list above at More Reading you might want to see FAN LIMIT SWITCH TROUBLESHOOTING for some diagnostics. It could be something as simple as the fan switch set to ON rather than AUTO - there are swtiches on many thermostats and on some limit switches. Another cause can be FAN ON wires from a thermostat that are incorrectly wired or are shorted together.

Question:

(Jan 4, 2015) R/M said:
This is a solution. Thank You
**************************
Reader Question: setting the fan limit switch FAN OFF down 5 degrees seemed to fix extra fan-on cycling - is this OK?

I notice that the fan on the heater came on briefly after the heating cycle concluded. I checked this article and decided to adjust the fan switch shut off temperature from 90 degrees to 85 degrees. So far the problem of an additional, short running fan cycle has not reoccurred. I've not read where this remedy is recommended. - Larry K 1/8/12

Reply:

Thanks for the comment Larry. Indeed in the article above we discuss the concern of blower fan cycling on and off one or more extra times at the end of a heating cycle. If that small shut-off temperature fixed the problem that's a great tip for other readers.

Just be sure that none of the more problematic reasons for fan cycling are the cause (as we describe above) and you should be ok

Reply:

(Jan 5, 2015) (mod) said:
R/M & Larry: thanks so much for the helpful follow-up. That will assist other readers.

Question: trouble setting or repairing a Camstat Time Delay Blower Control Model BC-7070 - set the TIME OFF dial

Fan limit switch BC-7070-A/M (C) InspectApedia.com(Jan 7, 2015) Chris said:

I am having a similar issue as several of these topics. The furnace will kick off then the blower will kick off shortly after. Then the blower will kick on briefly for a few seconds then off. It will do this repeatedly for 3-5 times. I did not see a similar switch in my furnace as what your pictures show.

I did take a pic of the Limit Switch, which is a BC-7070 AM. It has a Time On and Time Off adjustable dial (4 minutes). Should I increase the Time On dial so the blower runs longer after the furnace kicks off? It seems like this may blow the rest of the heat out which would prevent it from having to kick on/off.

Reply:

Chris

If this is a *change* in how the furnace operates then we want to diagnose and fix something, not just change a setting.

Often when a blower turns on and then off again then on again at the end of a furnace burner-on cycle it suggests that the residual heat in the heat exchanger is heating up air in the plenum enough to turn the blower fan back on again.

This might occur if the fan is not moving enough air (dirty air filter, clogged heat exchanger, obstructed air flow) or if the on-cycle is not long enough. If the air filters are not dirty and the ductwork not clogged, check also the blower fan itself for dirt or debris: a small amount of dirt on a blower fan - the squirrell cage type - can dramatically cut the air flow rate of the fan.

if none of those problems are present I'd try setting up the ON time to 5-6 minutes.

keep us posted as what you find will help other readers

(Jan 7, 2015) Chris said:

Hi Daniel-
Unfortunately I'm not sure if this is a "change" or not. It's really cold out now which is making the furnace run more which is who I've noticed this issue recently.

I did just buy this house a few months ago too which is another reason why I'm not sure if it's change. I have been good about changing the filters and will check for debris and dirt tonight. I have been reading more on Timer Delays. Isn't the TIME ON delay for when the blower kicks on after the furnace has kicked on? From what I've seen, I think I actually may want to adjust the TIME OFF delay to increase the time before the blower kicks off. Is this correct or am I mistaken? Once I resolve this issue I will be sure to update. Thanks!

(Jan 7, 2015) (mod) said:
Chris, we've posted some PDF's that describe the Honeywell Fan Limit Switch installation and operation - linked in the References to (for example) the FAN LIMIT CONTROL SETTINGS article found in More Reading above. But I'm not sure we cover your switch.

Your switch is a Camstat Time Delay Blower Control Model BC-7070 sold by various suppliers including A-1 Components Corp. The ON and OFF times can be adjusted between 0-240 seconds. The ON is the time delay before the fan starts after a call for heat. And as you suggest, the OFF time is the delay in turning off the fan AFTER the call for heat is satisfied.

We'll upload and link to a PDF with specifications for this control as well as other documents we can find when we move your comments into the article above. [ASAP]

Can you send me photos of your limit switch - use the email at our CONTACT US link found at page top or bottom and I'll research further.

(Jan 7, 2015) Chris said:
I've done some digging around and haven't found anything that covers my switch really. I have had a hard time even finding any instructions for this switch on the web. You're correct I have the one made by A-1 components. I am also emailing you a picture of the switch now. Thanks!

(Jan 7, 2015) (mod) said:
Chris by Email I've sent you the two PDFs of product info we have located so far.

Keep me posted

Daniel

Subsequent reply from Chris [by private email]

8 Jan 2015 Chris Said:

Thanks a lot for sending me these two documents Daniel. If I were to time when the blower fan kicks on and off and it doesn't correspond to the Delay switches at all, is it safe to say the limit switch is going out? There does appear to be quite a bit of dust in my squirrel cage. I was hesitant to change anything last night as it was the coldest day of the year so far

Moderator replied:

Chris I don't know this particular brand very well but I suspect you're right.

Do clean the fan if you can - I've measured airflow and seen a very big drop in air movement if the blades are dirty. They are just shallow-concave so it doesn't take much dust to flatten them thus reducing airflow. I'd like to see photos of the fan before and after cleaning.

15 January 2015 Chris said

Daniel-
I increased the Time Off dial and it fixed the problem. The extra time allows for the furnace to cool down enough that the blower doesn't continue to kick on. Thanks for the advice.

Reply:

Thanks for the feedback - it will assist others.

It's interesting that the approach taken by the manufacturer of your control relies on a simple timer setting. Other limit controls actually monitor the plenum temperature and turn the fan on or off accordingly. Your switch design is perhaps rugged, simple and economical but appears to eschew that more sophisticated approach.

Question: fan cycling on and off at end of heating cycle - non-adjustable limit switch

Mike said:
I have a similar issue, except I do not have a adjustable limit switch. After a heat cycle completes the fan stops and than it kicks in for a 1 second and than off again. At first it happened occasionally but now its every time. I checked filter and looked for clogs. My furnace is Bryant 394G it does not look like there is any way to adjust the fan timing. It has a time delay circuit that I think has pre-set time on it.

Reply:

Mike:

Make *certain* that there is good air flow through the heating plenum by replacing any air filters, checking for ductwork obstructions, and assuring that the squirrel cage blower fan is clean and working. If rthere is good air flow and you've got a non-adjustable limit switch my next step would be to replace the switch. There are on some non-adjustable limit switches a sealed snap switch that responds to temperature. If one of those fails, in theory it could be replaced but it's safer to just replace the fan limit switch assembly.

Question: furnace fan cycles during heat and one more time after burner turns off

(Mar 4, 2015) jeff said:
we installed a new motor to an older gas furnace. so the fan works now. however, it kicks on and off while the heat is on and after the heat has reached its temp. and kicks of the fan kicks off. then comes back on one last time stays on a min then goes back off. could you tell me what this might be.

Reply:

Jeff,

I'd start by asking if the new motor changed the fan speed. If not d look for a dirty air filter or dirty blower fan assembly that are,slowing air through the heat exchanger and plenum.

Question: Thermo Pride Furnace OC5, humming from fan relay

(Mar 14, 2015) Maxine Frese said:
We have a Thermo Pride oil furnace model OC5, direct drive. We replace the blower motor. Also, vacuumed the flue and clean out ports. We were very careful to label all the wiring on the motor when it was removed, so we know the wiring was plugged in correctly. We turned the power on and ran the furnace which started up fine. The furnace heated up and shut down correctly, the fan came on and shut off correctly. We thought everything was fixed, but then we heard a humming sound coming from the fan center relay area. Now, we never really noticed if the humming was there before we replaced the motor. It almost seem like there are contact stuck closed in the relay. We tapped on the relay but this didn't make a difference. Any thoughts on what the humming might be?

Reply:

CHeck for a bad start / run capacitor on the fan motor or for a bad relay.

Question: furnace blower fan runs by itself during summer

(May 26, 2015) P Dave said:
I have a 20+ year old gas furnace in the attic which is on top of 2nd floor. It works fine in the winter. In the summer, I keep my thermostate to "OFF" (neither Heat nor Cool) most of the time.
On a hot summer day, the blower fan in the furnace turns on (no heating or cooling, just the fan), even though thermostat is in OFF position. What adjustment is needed so that blower fan will stop running automatically in summer ?
Thanks. PD

(May 28, 2015) jpmorris55 said:
I have the same issue as P Dave, sort of. I have a 20-year-old oil-fired hot water system in the basement with forced air heat and air conditioning working off it. In the attic above the 2nd floor, there is another forced air system working off hot water pipes from the basement boiler (and a second air conditioning unit). The blower in the attic keeps going on every few minutes, even though the thermostat is switched to OFF. The only way to stop it is to turn off the power switch on the attic unit -- but then I can't use the air conditioning. I'd like to know what I'm dealing with before I call for service.

Reply:

It would be odd but not impossible for a plenum to get hot enough to turn on a blower fan even when the burner is off, but I'd look first for a loose wire or bad control board or for a thermostat FAN ON switch that's shorting-on.

Question: fan motor surfes at end of a cycle

(Dec 14, 2015) R jolliff said:
My HVAC fan motor lunges at the end of a cycle

Reply:

RJ

If you mean the fan is cycling on and off at the end of the heating cycle, please review the suggestions in the article above and let me know if questions remain.

If you mean the motor speeds-up, I suspect a control board or wiring failure.

Question: Rheem furnace with ECM blower, fan set to manual ON -but blower stops at start of call for heat

(Dec 17, 2015) magibson53 said:
I have a Rheem furnace with ecm blower motor. New system. I have set the fan to on during the winter to circulate air ( ceiling heat vents in house) to get a more even heat in the house. Also, to avoid the stop/start of the blower motor. The blower stop when the thermostat calls for heat and then comes back on after about 20 seconds. When the call for heat stops, system shuts down completely, and then the blower starts up again. Should the blower be stopping and starting when FAN is to ON?

Reply:

I suspect that your control thinks it's preventing blowing cold air into the occupied space. I don't know how to fix this system but I suspect it can be corrected by control board settings; I'd call my HVAC service manager to discuss the problem and then ask for help from an experienced Rheem furnace service technician.

(Dec 19, 2015) magibson53 said:
Installer said "On startup that how it's suppose to work. Needs to run tests before starting burners and kicking on the blower". Doesn't seem right to me.

Question: my furnace has no limit switch

(Dec 21, 2015) Linda said:
I was told my furnace has no limit switch (They say it's the circuit board) I don't know if I'm being told the truth. I never had this problem of of on and off during cool down until they put in new gas valve that did not fix the furnace. When they finally replaced thermostat (that finally made the furnace work and cycle right) this new problem started with this immediately.

They took $727.00 from me and now I have this problem. So stressed out. I have Comforter Maker RPJ II Model # GNJ075N12C1, Serial # L962216625. All everybody say's horrible furnace, I bought this home in 2012 and this is what I have. But I didn't need new gas valve, now I'm being told I need circuit board and God knows what else. I'm being robbed, I can't afford a new furnace. Does this furnace have a limit switch?

Again this was not the original problem and it never existed before this Hgt. & cooling place came here. They switched blower speed to high for what reason I don't know, the noise was horrible so second guy turned it back down. They also slammed the side of burners with pipe wrench (his way of cleaning pilot) 3 visits and I'm now stuck with this on/off back and forth in cooling mode. I can't sleep, this wakes me up. Verge of nervous breakdown. Can't trust another furnace person. Please help and give me some insight.

Reply:

Linda at the Continue Reading link above you'll see more images of Limit Switches so you can see what they look like. Even with a printed circuit board there must be a temperature sensor monitoring temperature in your furnace.

Every heating furnace has to have limit controls, otherwise the system would be unsafe and could not operate; but it is true that not all models use a field-adjustable switch. Some use a fixed temperature range sensor control.

If you are not confident about what a service tech is telling you it may make sense to ask for a second opinion from an independent company. Also be sure that your service tech is familiar with your particular equipment - that can reduce the chances of shotgunning approaches to repair: try replacing suspect parts. To be fair, often it's cheaper to replace a part than to do more extensive diagnostics - it depends.

Linda said:
This is kind of circuit board mine has (Note: this is the replacement for this model No.) mouse over picture enlarges, Please, can you tell me where the temperature sensor on it is. Can this alone be repaired without the whole thing being replaced. You can see what it can be purchased for not only here but locally, hgt. service wants over $400.00. I'm just wondering if the place that ripped me off could have messed with this or messed it up switching blower speeds.All your help is appreciated. It is the inducer fan not blower fan going up and down in cool down. I flipped the fan on thermostat from auto to on, and it was this fan going on/off. I beginning to wonder if the thermostat that put on wasn't a used defective one. All this is very confusing for the fact this was never a part of original problem.

Reply: problem was in the thermostat

Lijnda, By disconnecting the thermostat wires and joining them at one end then the other it's easy to determine if the wires are shorted or broken.

Reader follow-up:

(Dec 27, 2015) Linda said:
It was the Thermostat that rip off hgt. & cooling place put in. No doubt they knew it was defective trying to then sell me a circuit board I didn't need. I bought a new thermostat and had a friend put it in, has NOT made inducer fan go back on and off once during cool down. Normal thermo's heat one degree above what you set temp at, theirs never did. Fan probably kept coming back on and off because of it. Infuriating.

Reply:

Linda, Do you think perhaps the thermostat wa2miswired or a switch misplaced?

Question: blower runs blowing cool air for 10 minuts after burner is off

(Dec 30, 2015) Jay said:
My problem is that after the burner is off (the thermostat is satisfied), the blower keeps running for over 10 minutes blowing cool air. It is like the heater is on for some time with hot air and then the air conditioner is on for some time after the heater. It not only waste energy, but also don't make us feel comfortable. Does anyone know what is wrong and what we could do? Thanks! Wish everyone a happy New Year!

Reply:

Jay

Take a look at the fan limit control in the blower assembly: make note of what it does after the burner shuts off; if the plenum is very hot the blower will keep running until it cools down. Start by checking for a dirty air filter, inadequate return air, crimped ductwork.

Reader follow-up:

Thanks (mod) for your response and suggestion!
Actually I noticed just now that when I set a temperature, it started to heat with the blower on. However, when the temperature is satisfied, the blower didn't stop. Instead, it blows cold air after the burner is off and starts to blow hot air after a while and this cycle continues. We just changed the air filter and also the limit control. If we lower the temperature (e.g., 2 degrees lower) setting, the blower will stop in a minute or two.

Reply:

Usually the blower won't turn on until the plenum is warm enough, and the blower continues after the burner shuts off until the plenum has cooled down - to avoid damage to the heat exchanger as well as for efficiency. It shouldn't continue to blow cool air. Watching the dial on the limit switch might tell you if the switch is working normally and at what plenum temperatures it thinks it's responding. If the switch looks good I suspect a relay or control problem at the furnace.

It's easy to foul up a limit switch when changing its settings as if you don't handle it with care its spring can be bent. That'd leave it out of calibration. And maybe not so safe. It's also easy to mis-wire a thermostat or to mis-adjust its external and internal switches such as a heat anticipator.

(Dec 31, 2015) Jay said:
Hi (mod) , Thanks again!
How long do you think is normal for the blower to continue running after the blower shuts off? Is there a way to adjust the limit switch to control its responding time? Or maybe the new limit switch is accidentally out of calibration when installed so it is not functioning the right way? So we need another new limit switch?
Wish you a Happy New Year!

(Dec 31, 2015) Jay said:
Follow up: Finally we asked for help from a professional guy. He replaced the thermostat. It works normally now. Thanks (mod) for your comments and kind suggestion. He explained to me about the difference between the old and new thermostats, but I don't understand completely. He said the old thermostat is not for gas heater (if I understand correctly). Hopefully this will help someone later with similar problems.

Reply;

Thanks so much for the feed-back, Jay, that'll help other readers. Most thermostats work equally well for gas or oil fired heating boilers, but pehaps your service tech saw something we don't know about. The defect that would permit a thermostat to keep a blower ON when the thermostat is satisfied on a call for heat would be a a FAN ON switch set to ON or perhaps internal damage to that switch.

Question: blower won't turn off

(Jan 5, 2016) Robert said:
I have a older model split unit (house is an 89'). The heat works and the air works, but my problem is the blower wont turn off even after reaching set temp. I've been reading about the limit switch but there is not one on my unit at all and don't look like it ever had on. what could the problem be?

Reply:

Robert, search InspectApedia for BLOWER WONT TURN OFF to see an article that diagnoses and fixes the problem you describe.

Question: husband wired around the limit switch - do we need to change it

(Feb 10, 2016) Nicole said:
Hi our furnace wasn't blowing hot air, my husband thinks it a limit switch because some of the wires were loose and when he pushed on them the air turned hot again but only for a bit. So he replaced the wires and now we have heat. The only thing I am concerned about now is when the furnace kicks on, you can hear the blower but it takes a good minute for the air to come out, is this normal? Do we still need to change the limit switch? Please and thank you

Reply:

Nicole

If the blower is running but you have no heat the furnace itself is not heating. It sounds as if you found a loose wire.

Taking a minute for air to be felt at a supply register is a bit long; I'd look for a damaged or loose fan assembly.

(Feb 10, 2016) Nicole said:
Thank you so much! Do you think there is any concern for carbon monoxide problems with these issues?

Reply:

No Nicole, if the heat exchanger is not damaged (not leaky) fan cycling after heat is not unsafe.

However IF a limit switch is not set properly or is damaged and that allows the heat exchanger to overheat (burner runs but fan does not, for example) then the heat exchanger can crack and leak and thus become unsafe

Question: induction blower causing blower cycling ?

2016/03/21 larryinmich. said:
my intertherm mobile home furnace has an induction blower. could this have an effect on fan blower cycling after the stat is satisfied on the heat cycle?

Reply:

Larry I'm not sure what you're describing. If you mean that your furnace uses a draft inducer fan to vent the furnace oil or gas burner, that might indeed be drawing room air through the heat exchanger after the burner cut off. But that would cool DOWN the heat exchanger and plenum faster rather than more slowly, so it should reduce rather than increase the extra air-circulating blower-fan cycling on and off after the burner has turned off.

I'd start by checking the suggestions in the article FURNACE FAN CYCLES AFTER HEAT. Do keep me posted a what you find will help other readers.

HVAC Blower Fan Articles

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Or see BLOWER FAN OPERATION & TESTING - check for a dirty blower fan

Or see UNDERSIZED RETURN DUCTS - not enough return air

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